The present invention relates to the isolation of the facial stereoisomer of the electroluminescent molecule tris(8-oxoquinoline)aluminum(III) (Alq3), its mass production and its characterization in solution and in the solid state.
The first highly efficient and low-voltage-driven organic electroluminescent devices (OLED), have been reported by Tang and Van Slyke (1), and were based on Alq3. Fifteen years later, Alq3 is still a key electroluminescent compound, widely used in commercial devices and has become the prototype of a whole class of materials used as active layers in electroluminescent devices. Recently, significant improvements in device efficiency and stability have been obtained (2–6), and many efforts have been spent in order to extend and modify the typical green emission of Alq3-based OLEDs, using multilayers structures and chemical doping (7–9).
Tris-chelated octahedral complexes (such as Alq3) can exist in the fac or mer isomeric forms. In the case of trisoxoquinoline complexes (Mq3), only mer stereoisomers have been reported and characterized. The only reported example of a non-mer isomer is the Sbq3 complex (10) which, however, is not octahedral, thanks to the presence of a stereochemically active lone pair.
In the Alq3 molecule, the possible existence of different geometrical isomers is still an unresolved issue. Despite of the many investigation efforts during several years (11–14), the facial stereoisomer of Alq3 has never been directly observed. Invariably, spectroscopic studies on matrix-isolated molecules, solutions and polymorphic crystal phases (13, 15) have evidenced the existence of the green-emitting mer isomer only. Curioni et al. have theoretically predicted through computational models (16) that the fac isomer is less stable (ΔE≈4 kcal/mol) than the mer isomer and that it possesses a 0.3 eV higher energy gap (HOMO-LUMO), with a dipolar moment of 7 Debye (16).
Mer-Alq3 crystallizes as α e β phases (and in a number of clathrates), whose optical properties are determined by the nature of the π-π intramolecular contacts (15). In addition, partial crystallographic information on two phases, generated at elevated temperatures, called γ e δ, has been reported (15, 17).